Fuel pumps utilized for providing hydrocarbon fuels in liquid form to the fuel injectors of an internal combustion engine in an automobile or truck are usually powered by an electric motor in which the armature is mounted in the fuel pump body. These pumps must be capable of operating in a wide range of ambient temperatures.
Hydrocarbon fuels have a relatively low boiling point. In certain geographical areas, the ambient temperatures may reach 110.degree. to 120.degree. F. The temperature in the fuel tank below the automotive vehicle may be even higher than this. Since these pumps are frequently mounted in the fuel tanks, there is a great likelihood that the fuel in the pump may vaporize. The pumps are usually positive displacement pumps and it is necessary that the entry to the pump chambers create a low pressure to draw fuel into the pumping chambers.
This reduced pressure alone may cause a change in state of the fuel from liquid to vapor at elevated temperatures and significantly reduce the efficiency of the pump. In another condition, for example, when a vehicle has been operating and then the engine is shut off for a period, the fuel line between the pump and the fuel rail or fuel injectors is full of liquid fuel under pressure whereas the fuel in the pump can be completely vaporized due to the elevated temperature in the fuel tank and pump itself. Thus, when the engine is restarted, the pump is full of vapor and even the fuel in the pump intake filter may be vaporized. The pump cannot, under these conditions, generate enough pressure to move the fuel in the pressurized fuel supply line.
It has been previously known to provide a vapor port in a pump to bleed vapor from the high pressure area of the pump. These devices have been successful in expelling vapor from the pump. For example, U.S. Pat. No. 4,697,995 discloses such a purge port.
However, a noise problem still exists which has not been overcome. It is an inherent characteristic of a positive displacement pump to produce slight pressure pulses during its pumping cycle. Each pressure pulse causes expansion and contraction of the vapor within the purge port which causes an audible noise.